1. Technical Field
The present invention relates to a broadband matching network, and in particular to a laser communications system having a broadband impedance matching network.
2. Description of the Related Art
In conventional broadband communications systems, such as that depicted in FIG. 1, an amplifier 100 having a 75 .OMEGA. output impedance over a broad band of frequencies is applied to a conventional matching network 110. The conventional matching network 110 is comprised of two parallel combinations of a 56 .OMEGA. resistor 111, 112 and a 4.7 pF capacitor 413, 414 coupled between the amplifier 100 and a 75 .OMEGA. terminating resistor 120. A line between the two parallel combinations is coupled to a laser through a blocking capacitor 130. A direct current (DC) source is coupled to the laser through choke 140. The matching network 110 matches the output impedance of the amplifier 100 to the relatively low impedance of the laser. The resistors 111, 112 consume power wastefully. Accordingly, the amplifier 100 must output a relatively high output power to the matching network in order to drive the laser. Conventional amplifiers become nonlinear when driven to produce higher output power levels. As a result, the signal from one communication channel is degraded by signals from one or more other channels located elsewhere in the frequency band of interest. This is known as intermodulation distortion.
Furthermore, optical communication systems are becoming more expansive. Accordingly, the length of the optical trunk lines are increasing in order to extend service to distant areas. In addition, an increasing number of branch lines are split off from the trunk lines to reach the areas between the trunk lines. To transmit a given number of channels over the longer and increasingly split optical transmission lines, the carrier-to-noise ratio of the transmitted optical signal must be increased to compensate for attenuation. The carrier-to-noise ratio of the transmitted optical signal can be increased by increasing the output power of the amplifier signal. However, increasing the power output of the amplifier to improve the carrier-to-noise ratio also results in increased intermodulation distortion.
The light output power of the laser depends on the bias current applied to the laser and its slope efficiency. The slope efficiency, a characteristic of the particular laser used, is defined as the ratio of the amount of increase in light output power to the amount of increase of radio-frequency (RF) drive current applied thereto. Thus, a laser having a low slope efficiency requires an increased RF drive current to achieve a given output power. In order to produce the increased bias current, the amplifier must be driven to produce a relatively high output power. As above, producing such high output power causes intermodulation distortion in the output signal of the amplifier.
Complex amplifiers must be used to drive the lasers to avoid intermodulation distortion. However, these amplifiers are expensive both to make and to operate. While providing a linear output at relatively high output power levels, these amplifiers consume significantly more power than conventional amplifiers. The consumed power manifests itself as heat, which must be dissipated in order to prevent adverse effects on other components. As a result, the laser system must be provided with structure that dissipates the heat. It is well known from basic transmission theory that any impedance mismatch creates a point of reflection of an applied signal wave. The higher the degree of mismatch, the lower the return loss and the higher the level of reflected power. Some of these principles are described in C. L. Ruthroff, "Some Broad-Band Transformers," Proceedings of the IRE, August 1959, pp. 1137-1342. However, while the concept of broadband impedance matching is known, there remains a need in the art to properly design means for matching the impedance of a known laser device to the output of a radio frequency amplifier. Therefore, there exists a need in the field to provide broadband transmissions having high carrier-to-noise ratio and with low intermodulation distortion as inexpensively as possible.